Geophysical prospecting has been used extensively in the search for underground resources such as oil, gas, and minerals. Common techniques used for exploration include seismic, gravity, magnetic, and electrical methods. Seismic is historically the most widely used and can be subcategorized into seismic reflection and seismic refraction methods. With the seismic reflection method, the structure of subsurface formations is mapped by measuring the times required for a seismic wave, generated in the earth by a near-surface explosion, mechanical impact, vibration, or air gun, for example, to return to the surface after reflection from interfaces between formations having different physical properties. The reflections are recorded by detecting instruments responsive to ground motion or pressure waves. With reflection methods, one can locate and map, for example, such features as anticlines, faults, salt domes, and reefs.
The recorded data generally are processed using computers prior to being interpreted. The basic objective of seismic processing is to convert the information recorded in the field into a form that best facilitates geological interpretation. The field data are transformed into corrected record sections. One object of the processing is to eliminate or reduce noise. Another is to present the reflections with the greatest possible resolution.
Seismic sources may be placed in a wellbore is to provide seismic data that originates closer to the target formations of interest to enhance imaging and measurement accuracy and resolution. Borehole seismic/acoustic sources have utility in many applications including seismic imaging of the subsurface, time-lapse monitoring of reservoir production and production processes, reservoir stimulation, borehole cleaning, etc. In general, the more powerful the source signal, the greater the imaging distance and monitoring area, and the more effective the stimulation or cleaning effect. The limitation in source power output is ultimately the ability of the seismic or acoustic energy to damage the wellbore and/or the casing and cement lining the wellbore. A clamped borehole source, when compared with fluid-coupled sources, is uniquely useful in producing both compressional and shear wave data, reducing the tubewave production of the source, and increasing the output of the source, especially at low frequencies.
During the decade from the early 1980s to the early 1990s, many borehole source concepts were developed by researchers for oil and gas companies and oil and gas service companies. The potential for damage to the borehole is discussed, for example, in the article “Borehole Stresses Created by Downhole Seismic Sources”, Geophysics, 1991 by Winbow. Winbow teaches that many impulsive sources have a high likelihood of damaging cement behind casing in cased wells. One solution cited by Winbow is the use of fluid-coupled source such as piezoelectric sources. High-power clamped sources are specifically mentioned by Winbow as constrained in their output by considerations of cement damage.